the formation of vegetable mould-第20节
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earth as food instead of leaves； and when they are making deep
burrows。 This is rendered almost certain by the following weights
of the castings thrown up at the mouths of single burrows； the
whole of which appeared to have been ejected within no long time；
as was certainly the case in several instances。 The castings were
dried （excepting in one specified instance） by exposure during many
days to the sun or before a hot fire。
WEIGHT OF THE CASTINGS ACCUMULATED AT THE MOUTH OF A SINGLE BURROW。
（Weight in ounces given in parenthesisDP。）
（1。） Down； Kent （sub…soil red clay； full of flints； over…lying the
chalk）。 The largest casting which I could find on the flanks of a
steep valley； the sub…soil being here shallow。 In this one case；
the casting was not well dried （3。98）
（2。） Down。Largest casting which I could find （consisting chiefly
of calcareous matter）； on extremely poor pasture land at the bottom
of the valley mentioned under （1。） （3。87）
（3。） Down。A large casting； but not of unusual size； from a
nearly level field； poor pasture； laid down in a grass about 35
years before （1。22）
（4。） Down。 Average weight of 11 not large castings ejected on a
sloping surface on my lawn； after they had suffered some loss of
weight from being exposed during a considerable length of time to
rain （0。7）
（5。） Near Nice in France。Average weight of 12 castings of
ordinary dimensions； collected by Dr。 King on land which had not
been mown for a long time and where worms abounded； viz。； a lawn
protected by shrubberies near the sea； soil sandy and calcareous；
these castings had been exposed for some time to rain； before being
collected； and must have lost some weight by disintegration； but
they still retained their form （1。37）
（6。） The heaviest of the above twelve castings （1。76）
（7。） Lower Bengal。Average weight of 22 castings； collected by
Mr。 J。 Scott； and stated by him to have been thrown up in the
course of one or two nights （1。24）
（8。） The heaviest of the above 22 castings （2。09）
（9。） Nilgiri Mountains； S。 India； average weight of the 5 largest
castings collected by Dr。 King。 They had been exposed to the rain
of the last monsoon； and must have lost some weight （3。15）
（10。） The heaviest of the above 5 castings （4。34）
In this table we see that castings which had been ejected at the
mouth of the same burrow； and which in most cases appeared fresh
and always retained their vermiform configuration； generally
exceeded an ounce in weight after being dried； and sometimes nearly
equalled a quarter of a pound。 On the Nilgiri mountains one
casting even exceeded this latter weight。 The largest castings in
England were found on extremely poor pasture…land； and these； as
far as I have seen； are generally larger than those on land
producing a rich vegetation。 It would appear that worms have to
swallow a greater amount of earth on poor than on rich land； in
order to obtain sufficient nutriment。
With respect to the tower…like castings near Nice （Nos。 5 and 6 in
the above table）； Dr。 King often found five or six of them on a
square foot of surface； and these； judging from their average
weight； would have weighed together 7。5 ounces； so that the weight
of those on a square yard would have been 4 lb。 3。5 oz。 Dr。 King
collected； near the close of the year 1872； all the castings which
still retained their vermiform shape； whether broken down or not；
from a square foot； in a place abounding with worms； on the summit
of a bank； where no castings could have rolled down from above。
These castings must have been ejected； as he judged from their
appearance in reference to the rainy and dry periods near Nice；
within the previous five or six months； they weighed 9。5 oz。； or 5
lb。 5。5 oz。 per square yard。 After an interval of four months； Dr。
King collected all the castings subsequently ejected on the same
square foot of surface； and they weighed 2。5 oz。； or 1 lb。 6。5 oz。
per square yard。 Therefore within about ten months； or we will say
for safety's sake within a year； 12 oz。 of castings were thrown up
on this one square foot； or 6。75 pounds on the square yard； and
this would give 14。58 tons per acre。
In a field at the bottom of a valley in the chalk （see No。 2 in the
foregoing table）； a square yard was measured at a spot where very
large castings abounded； they appeared； however； almost equally
numerous in a few other places。 These castings； which retained
perfectly their vermiform shape； were collected； and they weighed
when partially dried； 1 lb。 13。5 oz。 This field had been rolled
with a heavy agricultural roller fifty…two days before； and this
would certainly have flattened every single casting on the land。
The weather had been very dry for two or three weeks before the day
of collection； so that not one casting appeared fresh or had been
recently ejected。 We may therefore assume that those which were
weighed had been ejected within； we will say； forty days from the
time when the field was rolled；that is； twelve days short of the
whole intervening period。 I had examined the same part of the
field shortly before it was rolled； and it then abounded with fresh
castings。 Worms do not work in dry weather during the summer； or
in winter during severe frosts。 If we assume that they work for
only half the yearthough this is too low an estimatethen the
worms in this field would eject during the year； 8。387 pounds per
square yard； or 18。12 tons per acre； assuming the whole surface to
be equally productive in castings。
In the foregoing cases some of the necessary data had to be
estimated； but in the two following cases the results are much more
trustworthy。 A lady； on whose accuracy I can implicitly rely；
offered to collect during a year all the castings thrown up on two
separate square yards； near Leith Hill Place； in Surrey。 The
amount collected was； however； somewhat less than that originally
ejected by the worms； for； as I have repeatedly observed； a good
deal of the finest earth is washed away； whenever castings are
thrown up during or shortly before heavy rain。 Small portions also
adhered to the surrounding blades of grass； and it required too
much time to detach every one of them。
On sandy soil； as in the present instance； castings are liable to
crumble after dry weather； and particles were thus often lost。 The
lady also occasionally left home for a week or two； and at such
times the castings must have suffered still greater loss from
exposure to the weather。 These losses were； however； compensated
to some extent by the collections having been made on one of the
squares for four days； and on the other square for two days more
than the year。
A space was selected （October 9th； 1870） for one of the squares on
a broad； grass…covered terrace； which had been mowed and swept
during many years。 It faced the south； but was shaded during part
of the day by trees。 It had been formed at least a century ago by
a great accumulation of small and large fragments of sandstone；
together with some sandy earth； rammed down level。 It is probable
that it was at first protected by being covered with turf。 This
terrace； judging from the number of castings on it； was rather
unfavourable for the existence of worms； in comparison with the
neighbouring fields and an upper terrace。 It was indeed surprising
that as many worms could live here as were seen； for on digging a
hole in this terrace； the black vegetable mould together with the
turf was only four inches in thickness； beneath which lay the level
surface of light…coloured sandy soil； with many fragments of
sandstone。 Before any castings were collected all the previously
existing ones were carefully removed。 The last day's collection
was on October 14th； 1871。 The castings were then well dried
before a fire； and they weighed exactly 3。5 lbs。 This would give
for an acre of similar land 7。56 tons of dry earth annually ejected
by worms。
The second square was marked on unenclosed common land； at a height
of about 700 ft。 above the sea； at some little distance from Leith
Hill Tower。 The surface was clothed with short； fine turf； and had
never been disturbed by the hand of man。 The spot selected
appeared neither particularly favourable nor the reverse for worms；
but I have often noticed that castings are especially abundant on
common land； and this may； perhaps； be attributed to the poorness
of the soil。 The vegetable mould was here between three and four
inches in thickness。 As this spot was at some distance from the
house where the lady lived； the castings were not collected at such
short intervals of time as those on the terrace； consequently the
loss of fine earth during rainy weather must have been greater in
this than in the last case。 The castings moreover were more sandy；
and in collecting them during dry weather they sometimes crumbled
into dust； and much was thus lost。 Therefore it is certain that
the worms brought up to the surface considerably more earth than
that which was collected。 The last collec
